CME Theory and Models bes, T G; Linker, JA; Chen, J ...
Space science reviews,
03/2006, Letnik:
123, Številka:
1-3
Journal Article
Recenzirano
This chapter provides an overview of current efforts in the theory and modeling of CMEs. Five key areas are discussed: (1) CME initiation; (2) CME evolution and propagation; (3) the structure of ...interplanetary CMEs derived from flux rope modeling; (4) CME shock formation in the inner corona; and (5) particle acceleration and transport at CME driven shocks. In the section on CME initiation three contemporary models are highlighted. Two of these focus on how energy stored in the coronal magnetic field can be released violently to drive CMEs. The third model assumes that CMEs can be directly driven by currents from below the photosphere. CMEs evolve considerably as they expand from the magnetically dominated lower corona into the advectively dominated solar wind. The section on evolution and propagation presents two approaches to the problem. One is primarily analytical and focuses on the key physical processes involved. The other is primarily numerical and illustrates the complexity of possible interactions between the CME and the ambient medium. The section on flux rope fitting reviews the accuracy and reliability of various methods. The section on shock formation considers the effect of the rapid decrease in the magnetic field and plasma density with height. Finally, in the section on particle acceleration and transport, some recent developments in the theory of diffusive particle acceleration at CME shocks are discussed. These include efforts to combine self-consistently the process of particle acceleration in the vicinity of the shock with the subsequent escape and transport of particles to distant regions.
Issue Title: Special Issue: Coronal Mass Ejections We present a brief introduction to the essential physics of coronal mass ejections as well as a review of theory and models of CME initiation, solar ...energetic particle (SEP) acceleration, and shock propagation. A brief review of the history of CME models demonstrates steady progress toward an understanding of CME initiation, but it is clear that the question of what initiates CMEs has still not been solved. For illustration, we focus on the flux cancellation model and the breakout model. We contrast the similarities and differences between these models, and we examine how their essential features compare with observations. We review the generation of shocks by CMEs. We also outline the theoretical ideas behind the origin of a gradual SEP event at the evolving CME-driven coronal/interplanetary shock and the origin of "impulsive" SEP events at flare sites of magnetic reconnection below CMEs. We argue that future developments in models require focused study of "campaign events" to best utilize the wealth of available CME and SEP observations.PUBLICATION ABSTRACT
Flux rope fitting (FRF) techniques are an invaluable tool for extracting information about the properties of a sub-class of coronal mass ejections (CMEs) in the solar wind. However, it has proven ...difficult to assess their accuracy since the underlying global structure of the CME cannot be independently determined from the data. In contrast, large-scale MHD simulations of CME evolution can provide both a global view as well as localized time series at specific points in space. In this study we apply five different fitting techniques to two hypothetical time series derived from MHD simulation results. Independent teams performed the analysis of the events in “blind tests”, for which no information, other than the time series, was provided. From the results, we infer the following: (1) Accuracy decreases markedly with increasingly glancing encounters; (2) Correct identification of the boundaries of the flux rope can be a significant limiter; and (3) Results from techniques that infer global morphology must be viewed with caution. In spite of these limitations, FRF techniques remain a useful tool for describing in situ observations of flux rope CMEs.
We consider a three-dimensional bipolar force-free magnetic field with a nonzero magnetic helicity, occupying a half-space, and study the problem of its evolution driven by an imposed photospheric ...flux decrease. For this specific setting of the Flux Cancellation Model describing coronal mass ejections occurring in active regions, we address the issues of the physical meaning of flux decrease, of the influence on field evolution of the size of the domain over which this decrease is imposed, and of the existence of an energetic criterion characterizing the possible onset of disruption of the configuration. We show that (1) the imposed flux disappearance can be interpreted in terms of transport of positive and negative fluxes toward the inversion line, where they get annihilated. (2) For the particular case actually computed, in which the initial state is quite sheared, the formation of a twisted flux rope and the subsequent global disruption of the configuration are obtained when the flux has decreased by only a modest amount over a limited part of the whole active region. (3) The disruption is produced when the magnetic energy becomes of the order of the decreasing energy of a semi-open field, and then before reaching the energy of the associated fully open field. This suggests that the mechanism leading to the disruption is nonequilibrium as in the case where flux is imposed to decrease over the whole region.
We propose a method for constructing approximate force-free equilibria in pre-emptive configurations in which a thin force-free flux rope is embedded into a locally bipolar-type potential magnetic ...field. The flux rope is assumed to have a circular-arc axis, a circular cross-section, and electric current that is either concentrated in a thin layer at the boundary of the rope or smoothly distributed across it with a maximum of the current density at the center. The entire solution is described in terms of the magnetic vector potential in order to facilitate the implementation of the method in numerical magnetohydrodynamic (MHD) codes that evolve the vector potential rather than the magnetic field itself. The parameters of the flux rope can be chosen so that its subsequent MHD relaxation under photospheric line-tied boundary conditions leads to nearly exact numerical equilibria. To show the capabilities of our method, we apply it to several cases with different ambient magnetic fields and internal flux-rope structures. These examples demonstrate that the proposed method is a useful tool for initializing data-driven simulations of solar eruptions.
A simple model of the coronal magnetic field prior to the coronal mass ejection (CME) eruption on 1997 May 12 is developed. First, the magnetic field is constructed by superimposing a large-scale ...background field and a localized bipolar field to model the active region (AR) in the current-free approximation. Second, this potential configuration is quasi-statically sheared by photospheric vortex motions applied to two flux concentrations of the AR. Third, the resulting force-free field is then evolved by canceling the photospheric magnetic flux with the help of an appropriate tangential electric field applied to the central part of the AR. To understand the structure of the modeled configuration, we use the field line mapping technique by generalizing it to spherical geometry. We demonstrate that the initial potential configuration contains a hyperbolic flux tube (HFT) which is a union of two intersecting quasi-separatrix layers. This HFT provides a partition of the closed magnetic flux between the AR and the global solar magnetic field. Such a partition is approximate since the entire flux distribution is perfectly continuous. The vortex motions applied to the AR interlock the field lines in the coronal volume to form additionally two new HFTs pinched into thin current layers. Reconnection in these current layers helps to redistribute the magnetic flux and current within the AR in the flux-cancellation phase. In this phase, a magnetic flux rope is formed together with a bald patch separatrix surface wrapping around the rope. Other important implications of the identified structural features of the modeled configuration are also discussed.
The Whole Sun Month campaign (August 10 to September 8, 1996) brought together a wide range of space‐based and ground‐based observations of the Sun and the interplanetary medium during solar minimum. ...The wealth of data collected provides a unique opportunity for testing coronal models. We develop a three‐dimensional magnetohydrodynamic (MHD) model of the solar corona (from 1 to 30 solar radii) applicable to the WSM time period, using measurements of the photospheric magnetic field as boundary conditions for the calculation. We compare results from the computation with daily and synoptic white‐light and emission images obtained from ground‐based observations and the SOHO spacecraft and with solar wind measurements from the Ulysses and WIND spacecraft. The results from the MHD computation show good overall agreement with coronal and interplanetary structures, including the position and shape of the streamer belt, coronal hole boundaries, and the heliospheric current sheet. From the model, we can infer the source locations of solar wind properties measured in interplanetary space. We find that the slow solar wind typically maps back to near the coronal hole boundary, while the fast solar wind maps to regions deeper within the coronal holes. Quantitative disagreements between the MHD model and observations for individual features observed during Whole Sun Month give insights into possible improvements to the model.
The Pre-CME Sun Gopalswamy, N.; Mikić, Z.; Maia, D. ...
Space science reviews,
03/2006, Letnik:
123, Številka:
1-3
Journal Article
Recenzirano
Issue Title: Special Issue: Coronal Mass Ejections The coronal mass ejection (CME) phenomenon occurs in closed magnetic field regions on the Sun such as active regions, filament regions, ...transequatorial interconnection regions, and complexes involving a combination of these. This chapter describes the current knowledge on these closed field structures and how they lead to CMEs. After describing the specific magnetic structures observed in the CME source region, we compare the substructures of CMEs to what is observed before eruption. Evolution of the closed magnetic structures in response to various photospheric motions over different time scales (convection, differential rotation, meridional circulation) somehow leads to the eruption. We describe this pre-eruption evolution and attempt to link them to the observed features of CMEs. Small-scale energetic signatures in the form of electron acceleration (signified by nonthermal radio bursts at metric wavelengths) and plasma heating (observed as compact soft X-ray brightening) may be indicative of impending CMEs. We survey these pre-eruptive energy releases using observations taken before and during the eruption of several CMEs. Finally, we discuss how the observations can be converted into useful inputs to numerical models that can describe the CME initiation.PUBLICATION ABSTRACT
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